In the mining industry, high voltage, usually multi-phase A-C power is supplied to mobile mining machines through multi-conductor cables which trail each machine. With mobile equipment, such as a mining machine, it is not practical to provide a ground at each location where the equipment operates. Therefore, each trailing cable usually includes, in addition to the current carrying conductors, at least one ground conductor connected to the frame of the mobile machine. The ground conductor connects the frame of the mobile machine positively to a ground at or near the power center which supplies the power to the machine.
To prevent hazards from shock at the mobile machines, the integrity of the ground must be maintained. Because of the environment in which the mobile machines operate, the ground wire is subject to damage and to faulty connection at the machine. If one of the current carrying conductors is shorted to the mining machine frame after the ground conductor breaks or its connection becomes faulty, a dangerously high voltage exists at the machine frame, and could harm mine personnel. For this reason, a continuous, reliable ground continuity monitoring system must be provided for each mobile machine.
Various devices to monitor the condition of the ground conductor connecting the mining machine to ground have been devised. Some monitoring systems include an extra conductor or pilot wire in the cable. A signal is provided to the remote machinery over the extra conductor and a different signal is transmitted back from the remote machinery through the ground conductor. Receipt of the different signal indicates continuity of the pilot wire and ground conductor circuit. Systems using an extra conductor are exemplified by the monitoring circuits of U.S. Pat. No. 3,335,324 -- Buckeridge, issued on Aug. 8, 1967 and of U.S. Pat. No. 3,496,416 -- Agnew, issued on Feb. 17, 1970. Buckeridge uses both ground and check conductors between the circuit breaker switch and a mining machine. Direct current in the monitoring circuit generated by a diode at the mining machine from the A-C power supply current provides operating current for the circuit breaker switch. A fault in the ground wire or check wire releases the switch terminating power to the remote unit. In the Agnew system, U.S. Pat. No. 3,496,416, a signal is sent over a pair of monitoring conductors in the cable to the mining vehicle. If there is continuity in the monitoring conductors, a signal transmitter is energized at the vehicle which transmits a signal to the source of power over the same conductors. Requiring the use of additional conductors or pilot wires, increases the expense of the system and may cause false operation of the monitoring system and unnecessary shut down of the equipment if the pilot wire should break while the ground wire remains intact. Also, the Agnew system requires a separate transmitter on each mining vehicle.
Examples of monitoring systems which do not use an extra conductor other than the ground wire include the apparatus of U.S. Pat. No. 3,728,582 of Thomas I. Agnew, issued on Apr. 17, 1973, U.S. Pat. No. 3,729,653 of Lawrence L. Manoff issued on Apr. 24, 1973, and U.S. Pat. No. 3,895,264 of Victor V. Kirilloff issued on July 15, 1975.
In the system of U.S. Pat. No. 3,728,582, the main source of power is disconnected from the cable when the apparent resistance of a conductor and the ground conductor exceeds a given value. Sensing the reflected impedance between the ground conductor and another conductor of the cable requires a transmitter circuit for each trailing cable in a multi-cable system. Each monitoring circuit must be isolated from other monitoring circuits operating in the system from the same power source.
In the system in U.S. Pat. No. 3,729,653, a D-C signal is impressed on the ground conductor by a transmitter on the mobile machine. When the current in the ground wire falls below a predetermined level, a relay is de-energized disconnecting the power from the mobile machine. In this system, a transmitter must be provided for each mobile machine. A diode bridge is required between the ground wire and the system ground. Access to the transmitter is limited because of its location upon the mobile machine rather than near the system power source.
The monitoring apparatus of U.S. Pat. No. 3,895,264 utilizes a star arrangement of impedances connected at the power source and at the mobile machine to provide a monitoring voltage which circulates current through the ground conductor and a monitoring impedance. One disadvantage of this system is that an impedance such as a solid state breakdown device must be inserted in the ground wire before it is grounded.